Thanks Brad and Ian for your help.
Brad, I did what you said, but if that were the problem, the speakers would be silent when headphones were connected, and in this case, they always sound, regardless if headphones are connected or not, so that didn't work, like I was afraid.
And Ian, seems you're right and it's definitely a hardware issue, something that only opening the board could be fixed. Wold be easy to open the board myself to take a look? I would do it in some days, after isolation ends.
To discard some weird software issue, I even did a reset to default settings, I didn't have any group of settings saved and in fact, I wanted to get rid off of some changes that didn't know how to revert, but the problem with the headphones remains.
Thanks again.